1. Field of the Invention
The present invention relates to a method of producing a solid wire for welding which has no copper plating. (In this specification, a simple term “wire” may be used in place of “solid wire”.)
2. Description of the Related Art
It is common practice to use a thin solid wire for welding (0.8-1.6 mm in diameter) in CO2 gas shield arc welding and MIG welding. When in use, the solid wire for welding is wound on a spool or placed in a pail pack. It is pulled out of the spool (or pail pack) by a feed roller and then pushed into a liner enclosed by a conduit cable and finally sent through the liner to the nozzle of the welding torch at the welding position.
The conduit liner mentioned above is a flexible guide tube formed from a spirally wound steel wire. It is usually 3-6 meters long and sometimes as long as 10-20 meters, depending on the distance to the welding position. The feeding of a wire for welding should be accomplished stably at a constant speed regardless of working places, where space may be limited or obstructed by steps or bends, as in a shipbuilding yard. Such stable feeding is one of the most important characteristic properties of a wire for welding, which is referred to as wire feedability.
When pushed into a conduit liner by a feed roller, a wire for welding undergoes feed resistance due to friction with the inside of the conduit liner. This feed resistance is not so large in a straight conduit liner as to pose problems with feedability. However, in the case of a long conduit liner or a sharply or repeatedly bent conduit liner, the feed resistance increases to such an extent that it does not balance with the feeding force any longer, resulting in the feedability deteriorating extremely.
Consequently, in order to ensure a stable feedability, it is necessary to reduce feed resistance due to friction with the conduit liner. A common way to reduce feed resistance and improve wire feedability is to coat the surface of a wire for welding with copper plating or a lubricant. Incidentally, copper plating produces an additional effect of greatly improving drawability, conductivity, and rust prevention.
Unfortunately, a solid wire for welding with copper plating or lubricant coating has drawbacks. That is, excess lubricant is easily scraped off as the solid wire is pressed by the feed roller. Residues of scraped lubricant enter the conduit liner and accumulate therein, which leads to an anomalous increase in feed resistance. Copper plating also causes a similar trouble. Residues scraped off from the copper plating pose a serious problem with an extreme increase in feed resistance. Moreover, the cyan compound used for copper plating is a source of environmental pollution. Under these circumstances, there has been a strong demand for a solid wire for welding which has no copper plating.
Copper plating on a solid wire for welding functions as a lubricating coating to improve feedability as well as drawability in the wire manufacturing process. Therefore, efficient drawing of a solid wire for welding which has no copper plating needs a good lubricant that supersedes copper plating or an improved drawing technology which obviates the necessity of copper plating.
The thin solid wire for welding as mentioned above is produced from a large-diameter wire (“stock” for short hereinafter) by passing it through a series of hole dies or roller dies. The above-mentioned technology to obviate the necessity of copper plating is disclosed in, for example, Japanese Patent Laid-open No. Hei-10-296488. It specifies the reduction of area for the drawing die, thereby preventing the wire breakage that occurs during high-speed drawing. Another way to prevent an anomalous increase in feed resistance has been proposed in, for example, Japanese Patent Laid-open No. 2000-117486. It relies on an improved lubricant which firmly adheres to the wire.
In production of a solid wire for welding which has no copper plating, the stock to be drawn needs surface treatment, such as bonderizing and boralization, which ensures good adhesion between the wire surface and the drawing lubricant. Such surface treatment poses problems with production cost and environmental pollution. Among common lubricants is calcium soap, which is a dry solid lubricant. Unfortunately, since calcium produces an extremely adverse effect on arc stability, calcium soap has to be removed by annealing, alkali degreasing, pickling, and washing with an organic detergent after drawing (to the final diameter or intermediate diameter). This also poses problems with production cost and environmental pollution.
On the other hand, a solid wire for CO2 gas shield arc welding suffers difficulties in firmly holding a predetermined amount of lubricating oil if it has a compact smooth surface. This problem is addressed by slightly roughening the wire surface. A surface-roughened wire permits feeding lubricant oil to be coated uniformly in its lengthwise direction. See Japanese Patent Laid-open No. 2000-107881.
A variety of similar technologies have been proposed. One of them is designed to interpose roller die drawing between dry hole die drawing and wet hole die drawing, thereby reducing the reduction of area to be achieved by wet drawing. Its object is achieved by roughening the surface of the finished wire to such an extent that surface irregularities firmly hold the feeding lubricant. See U.S. Pat. No. 6,079,243.
There has been proposed a method of transferring previously formed surface irregularities on a roller die to the wire surface. The finished wire effectively retains the feeding lubricant. See Japanese Patent Laid-open No. Hei-10-249576.
There has been proposed a technology for improving drawability while maintaining adequate surface roughness. This object is achieved by using MoS2 and WS2 as the drawing lubricant and specifying the reduction of area due to hole die drawing. This technology is based on the relationship between the area of reduction due to hole die drawing and the amount of lubricant adhering to the wire. See Japanese Patent Laid-open No. Hei-11-197878.
There has also been proposed a technology for causing potassium carboxylate to adhere to the wire surface in order to improve the arc stability. This object is achieved by washing the drawn wire with hot water and then subjecting the washed wire once to roller die drawing or roller pressing. See Japanese Patent Publication No. Hei-3-77035.
The foregoing technologies, however, are not practicable for a solid wire for welding which has no copper plating. In other words, they are incapable of providing the wire surface with irregularities large enough for them to hold sufficient lubricating oil, while maintaining the efficient wire productivity.
The disadvantage of a solid wire for welding which has no copper plating is its incapability of high-speed drawing through a hole die for the following reason. Hole die drawing, which is predominant in production of solid wires for welding, applies a much larger shear force to the lubricant layer than roller die drawing, and the shear force breaks the lubricant film. This problem readily arises when hole die drawing is carried out without the wire being treated with a lubricant of calcium stearate.
The same problem as above is also involved in the above-mentioned two-stage drawing process, which consists of dry hole die drawing and wet drawing. This drawing process is not applicable to high-speed drawing of a wire without copper plating because wet drawing is inferior to dry drawing in drawability. Moreover, as described in U.S. Pat. No. 6,079,243-given above, the surface roughness of a wire rapidly decreases during drawing, and hence it is difficult for the wire to maintain its surface roughness throughout the drawing process from the semi-finished stage (in which surface irregularities are made) to the final stage.
The above-mentioned MoS2 and WS2 as the drawing lubricant do not provide sufficient drawability in hole die drawing and roller die drawing, which makes it difficult to perform high-speed drawing on a wire without copper plating. Moreover, such lubricants remaining in excess amounts make unstable electric current flowing through the finished wire and residues of scraped lubricants aggravate the wire feedability. Therefore, it is necessary that the lubricant be applied uniformly, allowed to remain uniformly, and finally washed off uniformly. These problems lead to inefficient drawing and unstable quality. In addition, direct application of sulfides (such as MoS2 and WS2), without adequate rust-preventive treatment on the stock wire, deteriorates the corrosion resistance of welding wire.
The disadvantage of transferring surface irregularities to the wire surface from a roller die is that it is very difficult to control the roller surface which is subject to wear. This leads to unstable wire quality and high production cost due to frequent roller replacement.
The roller die is more suitable to high-speed drawing than the hole die, because it is made up of a pair of facing rollers which grip the wire. Because of this structure, the roller die applies a smaller shearing force to the lubricant layer during drawing than the hole die and hence causes the breakage of lubricant film less frequently. Moreover, the roller die is less subject to clogging than the hole die even in the case of drawing with a hydrogen-free inorganic solid lubricant which will not pose a problem with hydrogen increase. It would be possible to greatly improve the drawing speed and efficiency if the roller die is mainly employed to draw a welding solid wire free of copper plating.
However, the roller die has never been used to draw a welding solid wire free of copper plating at a high speed from the stock to the finished product. Drawing a welding solid wire free of copper plating without adverse effects on welding performance is limited in speed so long as it relies only on roller dies. High-speed drawing with roller dies presents more difficulties in the case of high-strength steel welding wire free of copper plating than in the case of ordinary mild steel welding wire. (The high-strength steel includes high-tension steel, alloy steel, and stainless steel, which are selected according to the work to be welded.)
As compared with mild steel welding wires, high-strength steel welding wires need a larger drawing force and hence exerts a larger load on the roller dies. With a large working force applied during drawing, the roller dies are subject to deformation and vibration, because the roller dies and their supporting frames are limited in strength and stiffness. This vibration causes the welding wire to come into contact with the roller die intermittently, thereby leaving minute dents on the wire surface. Such dents aggravate the accuracy of wire diameter and lead to the surface roughening of the wire. To avoid this problem, it is necessary reduce the drawing speed, even though drawing with roller dies is practicable. This leads to a low drawing efficiency and a low wire productivity.
There has been no instance in which drawing is substantially accomplished by means of roller dies only from the stock wire to the finished product. Even though such an instance exists, it would be impossible to draw a high-strength welding solid wire at a high speed.
The limitation of drawing speed is attributable to the roller dies which wear out and pose a problem with lubrication (as in the hole die). In the drawing of a welding wire free of copper plating, the stock wire may need surface treatment (such as bonderizing and boralization) for better adhesion between the wire surface and the lubricant. If this surface treatment is omitted, it is necessary to use a dry solid lubricant, such as calcium soap which provides good lubricity. Unfortunately, calcium soap remaining on the surface of the finished welding wire, however small the amount may be, greatly aggravates the arc stability. For this reason, a separate washing process is required to thoroughly remove calcium soap after drawing is completed. The result is high production cost and environmental pollution. Therefore, the use of calcium soap is detrimental to production of a welding wire free of copper plating.
For the reasons mentioned above, there has existed no in-line process for continuous production of a welding wire free of copper plating, said process including the steps of drawing, washing out lubricants for drawing, and applying lubricants for wire feeding. Even though such a process exists, it would be impossible to carry out these steps at the same high speed as drawing. Incidentally, the term “in-line” means continuous operation of drawing and its subsequent steps.
As mentioned above, in production of welding wires (including high-strength ones) free of copper, drawing from stock wires to finished wires has never been performed only by means of roller dies. Moreover, welding wires have never been produced by a continuous process including drawing, washing to remove drawing lubricants, and application of feeding lubricants. Even though such a continuous process exists, it would have been impossible to run it at a high speed.